1. Field of Invention
The present invention relates to a liquid crystal display. More particularly, the present invention relates to a multi-domain vertically aligned (MVA) liquid crystal display.
2. Description of Related Art
A conventional multi-domain vertically aligned liquid crystal display consists of an upper substrate board and a lower substrate board facing each other and parallel to each other. A transparent electrode is formed on the upper surface of the lower substrate board while another electrode is formed on the lower surface of the upper substrate board. A lower polarizing panel is attached to the underside of the lower substrate board while an upper polarizing panel is attached to top of the upper substrate board. The combination of the upper and lower polar panel permits only linearly polarized light in the orthogonal axis to pass through. A plurality of club-shaped liquid crystal molecules fills the space between the upper and the lower substrate board. The liquid crystal molecules are aligned in a direction perpendicular to the substrate boards.
When no external voltage is applied to the upper and lower electrodes, a beam of light entering the lower polarizing panel is linearly polarized. Since the long axis of the liquid crystal molecules is parallel to the direction of propagation of the light, there are no multiple twistings of the linearly polarized light. In other words, the linearly polarized light is unable to pass through the upper polarizing panel and so the area is dark. When an electric potential is applied between the upper and the lower electrodes, an electric field perpendicular to the substrate boards is produced. If the potential is greater than a threshold value, the electric field may be strong enough to rotate the liquid crystal molecules. Ultimately, the long axis of the liquid crystal molecules rotates to a fixed angle relative to the direction of the applied electric field. Hence, the linearly polarized light, subjected to the multiple twisting of rotated liquid crystal molecules, emerges as an elliptically polarized beam of light. Consequently, a portion of the incoming light is able to penetrate the upper polarizing panel to become a bright region.
In the multi-domain vertically aligned liquid crystal display, the long axes of the liquid crystal molecules are parallel to the electric field when the electric field is first established. Thus, their rotation rate is relatively slow at first. After the liquid crystal molecules have rotated for some time so that the long axes of the liquid crystal molecules are closer to the perpendicular direction of the electric field, their rotation rate increases considerably. To reduce the response time of the liquid crystal molecules and increase sensitivity, pre-tilt control is normally incorporated into a liquid crystal display. In other words, the long axes of the liquid crystal molecules are purposely positioned so that they are tilted at an angle relative to the direction of the applied electric field. In general, pre-tilting of molecules is achieved by forming slits or protrusions or a combination of the two on the color filter (CF) and the thin film transistor (TFT). By the introduction of these slits and protrusions, the long axes of a portion of the liquid crystal molecules are aligned tilted at an angle and some of the electric field lines are twisted.
FIG. 1 is a schematic cross-sectional view of a conventional multi-domain vertically aligned liquid crystal display having slits in the lower substrate board. As shown in FIG. 1, the lower substrate board 100 has a plurality of slits 106 therein. Most of the liquid crystal molecules 104 have their long axes aligned in a direction perpendicular to the upper and the lower substrate board. However, the long axes of those liquid crystal molecules 104 close to the slits 106 are tilted at an angle relative to the lower substrate board 100. When an external voltage of 7 V is applied to the electrodes, transparency rating of the liquid crystal is about 46% while the response time is about 20 msec. When an external voltage of 5 V is applied, the transparency rating of the liquid crystal decreases to about 43% while the response time increases to about 43 msec. Finally, if an external voltage of 3 V is applied, the transparency rating of the liquid crystal decreases considerably to about 15% while the response time increases considerably to about 179 msec.
FIG. 2 is a schematic cross-sectional view of a conventional multi-domain vertically aligned liquid crystal display having alternately positioned slits in both the upper and the lower substrate board. As shown in FIG. 2, both the upper substrate board 102 and the lower substrate board 100 contains a plurality of slits 106. Furthermore, the slits 106 in the upper substrate board 102 are alternately positioned with respect to the slits 106 in the lower substrate board 100. Most of the liquid crystal molecules 104 have their long axes aligned in a direction perpendicular to the upper and the lower substrate board. However, the long axes of those liquid crystal molecules 104 close to the slits 106 are tilted at an angle relative to the upper substrate board 102 and the lower substrate board 100. When an external voltage of 7 V is applied to the electrodes, transparency rating of the liquid crystal is about 44.15% while the response time is about 13 msec. When an external voltage of 5 V is applied, the transparency rating of the liquid crystal decreases to about 41.11% while the response time increases to about 27 msec. Finally, if an external voltage of 3 V is applied, the transparency rating of the liquid crystal decreases considerably to about 15.20% while the response time increases considerably to about 129 msec.
Accordingly, the additional slits in the upper substrate board 102 are able to reduce the response time of the liquid crystal by about one-third with little effect on the transparency ratings. However, the addition of slits in the upper substrate board 102 increases the number of processing steps and hence production cost.
FIG. 3 is a schematic cross-sectional view of a conventional multi-domain vertically aligned liquid crystal display having bumps on the lower substrate board. As shown in FIG. 3, the lower substrate board 100 has a plurality of bumps. An indium tin oxide (ITO) layer is formed over the bump to form a trapezoidal transparent electrode 108. In general, the long axes of the liquid crystal molecules 110 near the ITO electrode 108 are perpendicular to the surface of the ITO electrode 108. Moreover, the long axes of the molecules 104 also align with the electric field 112. Consequently, this portion of the liquid crystal molecules 104 cannot achieve the desired pre-tilt effect. Although the alignment of long axes of the liquid crystal molecules 104 differs more from the alignment of electric field 112 when the molecules are further away from the ITO electrode 108, the difference is only minimal. Hence, the pre-tilt effect resulting from this design is small.
Accordingly, one object of the present invention is to provide a multi-domain vertically aligned liquid crystal display having a high transparency rating and sensitivity.
A second object of this invention is to provide a multi-domain vertically aligned liquid crystal display that demands special processing of the electrode on the lower substrate board using conventional methods only.
To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention provides a multi-domain vertically aligned liquid crystal display. The multi-domain vertically aligned liquid crystal display includes a lower board, an upper board and a liquid crystal. The lower substrate board contains a plurality of slits therein. There is a bent protrusion structure on the surface between the slits. The bent protrusion structure has two pairs of symmetrical surfaces. The pair of surfaces next to the slits makes a first angle with the horizontal while the other pair of surfaces next to the mid-line makes a second angle with the horizontal. A thin film transistor is embedded in the lower substrate board underneath the bent protrusion structures for providing an electric field. An indium-tin-oxide electrode is formed on top of each bent protrusion structure. The upper substrate board is mounted on top and in parallel to the lower substrate board. The liquid crystal fills the space between the upper and the lower substrate board. The long axes of most of the liquid crystal molecules inside liquid crystal are perpendicular to the upper substrate board. The long axes of most liquid crystal molecules near the slits are perpendicular to the electric field. The long axes of most liquid crystal molecules above the bent protrusion structure deviate from the direction of the electric field by varying degrees.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.